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Mozilla last week said that slower-than-expected progress on Firefox for Microsoft's Windows 8 "Modern" user interface (UI) means that the touch-based browser likely won't launch until late January.

Estimates a month ago by the open-source developer had pegged Dec. 10 as the target release for the "Metro-ized" version of Firefox. Metro was the name Microsoft once applied to the Windows 8 and Windows RT radical UI, but the company ditched the moniker in August 2012 over a trademark dispute with a German retailer.

The Firefox "Preview Release" will still be packaged with the Aurora build of Firefox 26 for the Windows desktop when it launches Tuesday, as originally planned.

"The goal of the Metro Preview Release is to gather feedback and defects from the larger Aurora community," Mozilla said in notes published on its website from a planning meeting last Wednesday. "Whether or not the Metro Preview Release will graduate from Aurora to Beta and Release channels is still to be determined. At a minimum, it should significantly increase the number of testers and feedback."

Mozilla maintains three public build channels simultaneously: Aurora is the least polished and a precursor to Beta, which in turns leads to Release.

Aurora is to graduate Tuesday to version 26, when Beta and Release shift to versions 25 and 24, respectively.

Even though it will appear in Firefox 26's Aurora on Tuesday, a slowdown in development will probably postpone the appearance of Firefox Metro in the Release build to late January, Mozilla said. "The decrease in average team velocity over the two previous iterations positions Metro for a Firefox 27 release on January 21, 2014," the meeting notes stated.

The later release was not completely unexpected: Mozilla has pushed back the launch several times.

Mozilla started work on a Modern, previously known as Metro edition of Firefox 18 months ago. It shipped a rough preview in October 2012, several weeks before Microsoft shipped Windows 8. At that time, Mozilla's schedule said the Firefox app might appear as early as January 2013. In May, however, the company said its developers would complete Firefox for Modern between Oct. 2, 2013, and March 20, 2014, with mid-November the likeliest date.

Third-party browser makers face unique hurdles in Windows 8. Only the default browser -- which is set by the user -- can run in the Modern UI. During setup, Windows 8 assigns Internet Explorer 10 (IE10) as the default browser; when Windows 8.1 ships Oct. 18, IE11 will be set as the default.

Even so, Mozilla has said it's important that it have a Modern browser app to remain competitive. Google, for instance, has had a much less ambitious Modern version of Chrome since June 2012.

Mozilla will launch the Release version of Firefox 24 for Windows, OS X and Linux desktops early Tuesday. Among the expected changes will be support for OS X's vanishing scrollbars -- a feature Apple introduced in mid-2011's Lion -- and the ability to close all tabs to the right of a selected tab.